Does aspartic acid racemization constrain the depth limit of the subsurface biosphere?

Abstract

Previous studies of the subsurface biosphere have deduced average cellular doubling times of hundreds to thousands of years based upon geochemical models. We have directly constrained the in situ average cellular protein turnover or doubling times for metabolically active micro-organisms based on cellular amino acid abundances, D/L values of cellular aspartic acid, and the in vivo aspartic acid racemization rate. Application of this method to planktonic microbial communities collected from deep fractures in South Africa yielded maximum cellular amino acid turnover times of ~89 years for 1 km depth and 27 °C and 1-2 years for 3 km depth and 54 °C. The latter turnover times are much shorter than previously estimated cellular turnover times based upon geochemical arguments. The aspartic acid racemization rate at higher temperatures yields cellular protein doubling times that are consistent with the survival times of hyperthermophilic strains and predicts that at temperatures of 85 °C, cells must replace proteins every couple of days to maintain enzymatic activity. Such a high maintenance requirement may be the principal limit on the abundance of living micro-organisms in the deep, hot subsurface biosphere, as well as a potential limit on their activity. The measurement of the D/L of aspartic acid in biological samples is a potentially powerful tool for deep, fractured continental and oceanic crustal settings where geochemical models of carbon turnover times are poorly constrained. Experimental observations on the racemization rates of aspartic acid in living thermophiles and hyperthermophiles could test this hypothesis. The development of corrections for cell wall peptides and spores will be required, however, to improve the accuracy of these estimates for environmental samples.

An oligotrophic deep-subsurface community dependent on syntrophy is dominated by sulfur-driven autotrophic denitrifiers

“…The concentrations of the atmospheric noble gas isotopes (72) ( 36 Ar, 84 Kr, 86 Kr, and 129 Xe) are a fraction of those for groundwater in equilibrium with the atmosphere at the mean annual temperature for central South Africa. This indicates that the fracture water has lost some of its dissolved gas, presumably to the head space of the fractures during dewatering of the fractures by the mining operations.…”

“…This indicates that the fracture water has lost some of its dissolved gas, presumably to the head space of the fractures during dewatering of the fractures by the mining operations. We used the diffusion model of Lippmann et al (25) and the concentrations of 36 Ar, 84 Kr, 86 Kr, and Calculation of Free Energy Fluxes. Steady-state free energy fluxes of 37 energyyielding redox reactions for autotrophy, heterotrophy, methanotrophy, and methanogenesis were estimated as described in Onstott (75) and Magnabosco et al (4).…”

“…For simplicity, we make the same assumption in our model. Onstott et al (86) and Hoehler and Jørgensen (87) point out that the maintenance energy values from Eq. 9 appear to be two to three orders of magnitude greater than retentostat measurement of the maintenance energy rate (88), but to match the observed cellular abundance calculated from our model with the observed cellular concentration we only had to decrease the preexponential term from 4.5 to 4.5 × 10 −1…”

A metagenomic window into carbon metabolism at 3 km depth in Precambrian continental crust

“…A recent reevaluation of protein turnover times in the deep subsurface derived from the D/L aspartic acid racemization ratio estimated the maximum protein turnover times for the Witwatersrand Basin microbial communities to be 1-2 years (Onstott et al, 2014). This finding indicates that the deep crustal biosphere is metabolically more active than previously thought, so much so that the low DOC concentrations in the Witwatersrand Basin fracture water would be depleted in~10 3 years.…”

Microbial life in deep subseafloor coal beds

“…The energetic cost of amino acid racemization and the resulting inactivation of proteins have been suggested to restrict microbial life in low-energy environments to an upper temperature limit of approximately 85°C (17). This limit may, however, have been exceeded during a recent expedition of the D/V Chikyu off the east coast of Japan.…”